It is known to measure the velocity of fluid flow in a duct by transmitting acoustic signals through the duct and measuring the time of propagation of the signals along the duct. For a known distance, the velocity can be calculated. Generally, measurements are taken in both directions along the duct to provide for increased accuracy of the result.
However, when a pulsed acoustic signal is propagated within a duct, the acoustic wave packet breaks into a number of transition modes each having a frequency corresponding to the transmitting frequency but having a difference phase whereby at different points along the duct, each mode has its own set of null points at which the amplitude of the mode wave is zero. Also, the different modes have different acoustic velocities.
In order to obtain flow measurements, the acoustic wave must be received by a transducer or like apparatus arranged at some point within the duct. The receive transducer however does not receive a single pulse of acoustic energy as was transmitted, but however, it receives the various modes each having a different energy level and being received at different times. Depending on the energy level of the received signal, it will be apparent that errors arise in determining a point in time in which a particular wave packet is received and hence a flow measurement can be determined. Because of this, the errors are carried over to the resulting flow velocity calculation.
It is an object of the present invention to substantially overcome or ameliorate, the abovementioned problems through provision of a method by which flow velocity errors can be substantially reduced.